Abstract

We report a significant improvement of the finesse and quality factor of a calcium fluoride whispering gallery mode optical resonator achieved with iterative thermal annealing. The initial and final values of the resonator finesse are ℱi ≈ 5 × 105 and ℱf ≥ 107 respectively. The measurements are conducted at room temperature in the laboratory atmosphere.

Figures (3)

Left: Ring-down signal after one annealing step. Right: Ring-down signal after three annealing steps. The exponential fit (red solid curve) is nonlinear in the logarithmic scale because the exponents have constant offsets. Keeping in mind that Q = ωτ/2, and ω = 2πc/λ, λ = 1.55 μm, we find that the values of the quality factors after the first and third annealing steps are Q = 1.2 × 1010 and Q = 6.7 × 1010 respectively. It is important to note that the initial value of the Q-factor corresponds to the earlier observations [10], while the final value is the apparent improvement at the given wavelength.

Left: Ringdown signal after the third annealing step taken with low-power laser radiation (blue solid line). The theoretical fit of the signal is shown by the red dotted line. Right: Evaluated change of the period of the signal. The solid line stands for the linear fit of the time dependence of the period of the ringdown signal.

Change of the signal amplitude shown in Fig. 2(left) with time. Initial and final quality factors are Q = (7.9±0.5) × 1010 and Q = (3±1) × 1011 respectively. Solid line stands for the theoretical fit of the ringdown signal found using formalism presented in [19]. A small oscillation of the decaying signal may result from the residual scattering in the material becoming observable at the given value of quality factor.